An optically anisotropic layer is formed through the steps of aligning discotic liquid crystal molecules and fixing the alignment. The discotic liquid crystal molecules give large birefringence, and can be oriented according to various alignment modes. An optical compensatory sheet obtained from the discotic molecules, therefore, has specific optical characters that cannot be given by a conventional stretched birefringent film. Molecular Crystals and Liquid Crystals, vol. 84(1982), pp. 193 discloses a discotic liquid crystal triphenylene compound showing negative birefringence. All the liquid crystal molecules should be evenly aligned in the optical compensatory sheet. In other words, the discotic liquid crystal molecules are preferably oriented in mono-domain alignment. However, conventional discotic liquid crystal molecules generally orient themselves in dual-domain alignment. Therefore, defects are often observed on the border of the two domains. Accordingly, conventional discotic liquid crystal molecules often do not show enough optical characters to be used in the optical compensatory sheet. The optical characters generally depend upon the structure of discotic liquid crystal molecule. Therefore, many discotic liquid crystal compounds have been studied and developed to obtain desired optical characters. For example, Japanese Patent Provisional Publication No. 8(1996)-50206 discloses an optical compensatory sheet comprising a transparent support and a thereon-provided optically anisotropic layer containing discotic liquid crystal molecules.
Japanese Patent Provisional Publication Nos. 7(1995)-306317 and 9(1997)-104866 disclose 2,3,6,7,10,11-hexa{4-(4-acryloyloxyhexyloxy)benzoyloxy}triphenylene as a discotic liquid crystal compound suitable for forming an optically anisotropic layer of optical compensatory sheet.
The retardation (Δnd) of the optical compensatory sheet is designed according to optical characters of the liquid crystal cell to be compensated. The value of retardation (Δnd) is a product of birefringent anisotropy (Δn) and thickness (d) of the optically anisotropic layer. Even if the thickness (d) is small, an optically anisotropic layer having sufficient birefringent anisotropy (Δn) can compensate the liquid crystal cell. However, it is very difficult to prepare the optically anisotropic layer having sufficient birefringent anisotropy (Δn) from known discotic liquid crystal compounds, for example, those disclosed in Japanese Patent Provisional Publication Nos. 7(1995)-306317 and 9(1997)-104866.
Japanese Patent Provisional Publication No. 2001-166147 discloses a discotic liquid crystal compound having large birefringent anisotropy. However, the disclosed compound has a poor optical character on wavelength dispersion. In other words, the compound enlarges the wavelength dispersion. Therefore, the resultant optical compensatory sheet cannot be sufficiently improved.
Generally, the character on wavelength dispersion inversely relates to birefringent anisotropy. In other words, the more the birefringent anisotropy is increased, the more the wavelength dispersion is usually impaired. If the wavelength dispersion is impaired, the optical compensatory sheet cannot fully prevent a displayed color image from changing chromaticity unfavorably. It has been desired to improve both the birefringent anisotropy and the character on wavelength dispersion at the same time.
It is known that phases formed by discotic liquid crystal compounds are roughly categorized into three phases, namely, a columnar phase, a discotic nematic (ND) phase, and a chiral discotic nematic phase. In the columnar phase, central cores of the discotic molecules are piled up in the form of columns through intermolecular force. In the discotic nematic phase, the discotic molecules aggregate randomly. W. H. de Jeu (Physical properties of liquid crystalline materials, Gordon and Breach Science Publishers, 1980) reports that the discotic nematic phase is seldom observed while the columnar phase is often observed. In addition, the discotic nematic phase of triphenylene compound has been observed only in the case that the triphenylene compound has benzoyloxy or cinnamoyl substituent groups at 2, 3, 6, 7, 10 and 11-positions.